1. Field of the Invention
The present invention relates to a method of producing a stamper used for producing an optical disc and a read-only optical disc production method.
[Patent Document 1] Japanese Patent Application Laid-open No. 2007-287261
[Patent Document 2] Japanese Patent Translation Publication No. 2003/009284
2. Description of the Related Art
In recent years, along with a densification of optical discs, a Blu-ray disc (registered trademark) is widely used as a high-density optical disc, for example.
While DVDs (Digital Versatile Discs) already in wide use each have a storage capacity of 4.7 GB (Giga Bytes) (single recording layer), Blu-ray discs each have a storage capacity as large as 25 GB. Such a densification is made possible by miniaturizing a pit pattern in a disc matrix mastering process.
In a mastering process of DVDs of the past, an organic resist subjected to a laser exposure has been sensitized by a photon mode. A photon-mode recording area is proportional to an exposure spot diameter and has a resolution almost equal to a half-value width of the spot diameter. The exposure spot diameter φ is expressed by φ=1.22*λ/NA, where λ represents a laser wavelength and NA represents a numerical aperture of a lens.
In contrast, in the mastering process of Blu-ray discs, cutting is performed with a resist that uses an inorganic material (inorganic resist) and with which a resolution can be significantly improved.
A read-only Blu-ray disc, that is, a ROM-type Blu-ray disc (hereinafter, also referred to as “BD-ROM”) onto which information is recorded by concavoconvex emboss pit patterns is produced as follows.
First, cutting of a matrix that uses an inorganic resist and development processing are carried out to thus obtain a matrix on which a pit pattern is formed. Then, using the matrix, stampers are formed by a nickel electroplating process. The stampers are obtained by inversely transferring the pit pattern of the matrix.
After that, plastic substrates (disc substrates) are formed by injection molding using the stampers, and a reflective film, a cover layer, and the like are formed on each of the formed disc substrates, to thus obtain a final optical disc.
Here, when producing optical disc products expected to be of a high demand as in a case of providing hit movies in discs, it may be necessary to efficiently produce an enormous amount of optical discs.
Therefore, in the case of BD-ROMs, a production method that enables a large number of stampers to be produced from a single matrix is demanded.
Unlike a matrix that uses an organic resist in the related art, the matrix that uses an inorganic resist is capable of producing a large number of stampers from a single matrix.
As shown in FIG. 7, if a large number of master stampers Ma (Ma#1, Ma#2, Ma#3, . . . ) can be formed from a single matrix, injection molding can be carried out in parallel using the master stampers Ma to thus produce disc substrates.
Further, each of the master stampers Ma is plated again, and mother stampers MT (MT#1, MT#2, . . . ) whose concavities and convexities are inverted from those of the master stampers Ma are thus formed. Then, son stampers SN (SN#1, SN#2, . . . ) whose concavities and convexities are further inverted from those of the mother stampers MT are formed. As a result, also by using the son stampers SN, disc substrates can be produced by carrying out the injection molding in parallel.
This is a desirable copying process for a mass production because, if a large amount of master stampers Ma and son stampers SN (i.e., stampers that can be used for injection molding) can be produced exponentially from a single matrix, disc substrates can be produced in parallel using those stampers, with the result that an improvement in efficiency of the whole process and a short-term mass production can both be realized.